1. Field of the Invention
The present invention relates to a driving method and a driving circuit for a piezoelectric transformer, a cold-cathode tube light-emitting apparatus using a cold-cathode tube as a load of a piezoelectric transformer in the driving method and the driving circuit, a liquid crystal panel in which the cold-cathode tube light-emitting apparatus is built, whereby brightness is controlled, and a device with a built-in liquid crystal panel, such as a mobile telephone, a portable information terminal (PDA: Personal Digital Assistant), a communication terminal, etc., in which the liquid crystal panel is built.
2. Description of the Related Art
Hereinafter, a conventional method for driving a piezoelectric transformer will be described.
The piezoelectric transformer has a configuration in which a primary (input) side electrode and a secondary (output) side electrode are formed on a piezoelectric material, an AC voltage in the vicinity of a resonance frequency of the piezoelectric transformer is applied to the primary side electrode to vibrate the piezoelectric transformer mechanically, and the mechanical vibration is converted by a piezoelectric effect so as to be output from the secondary side electrode. The piezoelectric transformer can be handled at an energy density higher than that of an electromagnetic transformer. Therefore, the piezoelectric transformer can be rendered smaller and thinner, compared with the electromagnetic transformer, so that a high conversion efficiency can be realized.
Furthermore, the piezoelectric transformer converts energy via electrical/mechanical conversion. Therefore, the electromagnetic noise of the piezoelectric transformer emitted to a space is much smaller than that of the electromagnetic transformer.
Generally, in the piezoelectric transformer, due to the impedance of a load connected to the secondary side, a voltage step-up ratio, which represents a ratio of a voltage output from the secondary side with respect to a voltage input to the primary side, is varied. Furthermore, a driving efficiency represented by the electric power output from the secondary side with respect to the electric power input to the primary side is varied similarly. Therefore, the driving frequency also is varied, which enables the maximum voltage step-up ratio and driving efficiency to be obtained. More specifically, in order to drive the piezoelectric transformer efficiently at a predetermined voltage step-up ratio, it is required to set the driving frequency in accordance with the impedance of a load to be connected.
For example, in the case of using a cold-cathode tube as a load of the piezoelectric transformer, the cold-cathode tube generally exhibits a high impedance equal to or more than hundreds of MΩ until it lights up, and the impedance decreases rapidly to a range between hundreds of kΩ and tens of kΩ after it lights up. Therefore, in order to allow the cold-cathode tube to light up efficiently by using the piezoelectric transformer, it is required to change the frequency and the level of a voltage applied to the primary side of the piezoelectric transformer between a period before the commencement of lighting and a period after lighting.
In the case of configuring an inverter circuit, using the piezoelectric transformer, a rectangular wave is formed at a frequency in the vicinity of a resonance frequency of the piezoelectric transformer, using at least one switching element. Furthermore, a filter circuit is provided between the output side of the switching element and the primary side of the piezoelectric transformer, whereby the piezoelectric transformer is driven under a condition of an input voltage of the piezoelectric transformer being approximated to a sine wave as closely as possible.
In order to enhance the conversion efficiency of the piezoelectric transformer, it is required to minimize the input of a frequency component, other than those for driving the piezoelectric transformer, to the piezoelectric transformer. In the case where an inverter circuit is configured using the piezoelectric transformer, since the piezoelectric transformer is a capacitive element, it is required to provide a filter circuit using an inductor between a switching element and a primary side electrode of the piezoelectric transformer according to a conventional driving method.
As a prior art for realizing the above, a power conversion apparatus (Conventional Example 1) as shown in FIG. 16A is known (or example, see page 5, FIGS. 1(b), 6, and 8 in JP 10(1998)-201241 A), and a power conversion apparatus (Conventional Example 2) as shown in FIG. 17 also is known (for example, see page 5, FIGS. 4, 5, and 9 in JP 10(1998)-201245 A).
Conventional Examples 1 and 2 are exemplary methods for driving a piezoelectric transformer with a stepped waveform signal, in which an inverter circuit is configured using a piezoelectric transformer.
In Conventional Example 1 shown in FIG. 16A, the timing of charging/discharging of capacitors C1, C2, and C3 is controlled by switching elements for charging S1, S2, S3, S4, and S5, and switching elements for discharging S6, S7, S8, S9, and S10, whereby a voltage level is set from a D.C. power supply 103. A stepped voltage waveform in which the time of one step is W1 is generated by the setting of a voltage level and the timing of switching of the switching elements Sa, Sb, Sc, and Sc, as shown in FIG. 16B. The stepped voltage waveform is applied to the piezoelectric transformer 101, thereby supplying a load 102 with a power.
Furthermore, in Conventional Example 2 shown in FIG. 17, an inductor 104 is connected between a common connecting portion A of the switching elements Sa and Sb, and one of primary side electrodes of a piezoelectric transformer 101. A capacitor 105 is connected between both the primary side electrodes of the piezoelectric transformer 101. The inductor 104 and the capacitor 5 constitute a filter circuit, and shape the stepped waveform shown in FIG. 16B into a sine wave.
However, the above-mentioned Conventional Examples 1 and 2 merely disclose a method for driving a piezoelectric transformer with a stepped waveform signal.
There are a plurality of vibration modes of the piezoelectric transformer. Therefore, in the case where the piezoelectric transformer is driven with a driving waveform other than a sine wave, a reactive power is increased due to the capacitance component of an input part of the piezoelectric transformer, or a harmonic component excites a high-order vibration mode of the piezoelectric transformer.
Therefore, in the case of using a cold-cathode tube as a load of the piezoelectric transformer, it is required that an inverter circuit should handle a reactive power, as well as a power required for lighting up the cold-cathode tube. As a result, the efficiency of the inverter circuit decreases and the efficiency decreases due to the influence of a dielectric loss in the piezoelectric transformer, which makes it difficult to enhance a conversion efficiency.
Furthermore, when a high-order vibration mode of the piezoelectric transformer is excited with a harmonic component, distortion due to a high-order vibration mode superimposed on a desired vibration mode may be generated in the piezoelectric transformer. This causes a decrease in the withstand power of the piezoelectric transformer, and degradation of the characteristics due to a large amplitude operation, leading to a decrease in reliability.
The above-mentioned problem becomes an obstacle to the miniaturization of a piezoelectric inverter, and the miniaturization of communication equipment using a liquid crystal panel in which a light-emitting apparatus of a cold-cathode tube is built, using a piezoelectric inverter.